Sulu picks up a ship on sensors. The small craft is on course directly for the Denevan star, and does not appear to be out of control. Kirk orders a warp 8 interception course. The ship is out of range of the tractor beam; the Enterprise pursues. Finally, they make contact: seconds before the ship burns up, the pilot cries out "I did it! It's finally gone! I'm free!!".

Kirk forms a landing party consisting of him, Spock, McCoy, Scotty, YeomanZahra and Bobby. Once on the planet, they are struck by the curious lack of people; in a city of 100,000 people, no one is visible – until, a few minutes later, they are attacked by four men who, even as they charge, scream "Go away! We don't want to hurt you!" But, with their crude clubs, they try anyway, forcing the landing party to stun them, an attitude inconsistent with their actions. Then McCoy discovers that the nervous systems of the unconscious men are violently active – as if they are somehow still being stimulated.

A scream draws them next to Kirk's brother's lab. Captain Kirk's brother, Sam, lies dead on the floor. Aurelan, Sam's wife, is hysterical, and their child Peter is unconscious nearby. Evidence suggests something has been trying to force its way in, despite the fact that the sensors showed nothing on Deneva that didn't belong there.

Aurelan, in terrible pain, tells Kirk that "things" came, eight months prior, on a ship from Ingraham B. As she tries to answer Kirk's questions, she experiences more and more pain, until McCoy is forced to sedate her. The creatures use the Denevans as their arms and legs, and are forcing them to build ships. They control their hosts with pain. Aurelan's last act is to implore Kirk not to let the things go any further; this effort costs her everything she has left, and she dies.

Kirk rejoins the landing party; he knows there is some sort of creature present, but the landing party has not yet discovered anything beyond a curious buzzing. Entering a building where they heard this sound, the landing party discovers strange creatures. Looking like little more than loathsome blobs of jelly, they emit an unwholesome buzzing, and employ a crude, wingless flight. A phaser at force 3 – sufficient to destroy most organisms – barely affects these creatures, even after several seconds of exposure. And the creatures do not register on Spock's tricorder.

Kirk orders the landing party out of the infested area; as they leave, a creature strikes Spock, leaving a strange puncture wound.

McCoy removes a small strand of tissue, and then, over NurseChapel's objections, he closes the wound. The creatures attack by stinging; they leave behind a piece of this tissue that rapidly infiltrates the victim's entire nervous system, far too completely for conventional surgery to remove it.

Spock recovers consciousness, rages out of sickbay and storms the bridge. His goal: to take the ship out of orbit. Spock is ultimately overcome and returned to sickbay, where McCoy makes another grim discovery. The K3 indicator, a measure of pain, is very, very high. The reason for the madness is confirmed: victims are in such agony that their minds eventually break under the stress. Spock, recovering consciousness, now claims the ability to control the pain. But after his visit to the bridge, Kirk isn't sure and has him continued to be confined to sickbay.

Spock, conquering the pain, breaks out of sickbay and plans to visit the planet's surface. Scotty, acting on Kirk's orders, refuses to transport him. A scuffle breaks out, including Spock using the Vulcan nerve pinch on Leslie and when Kirk appears, Spock explains that his plan is to retrieve a creature for study. He believes that since his nervous system is already infiltrated, there is little more the creatures can do to him. Kirk is convinced, and over McCoy's objections, Spock beams down to collect a creature for study.

Spock returns with a creature and begins to study it. Immediately, he realizes that the creature resembles, more than anything, an enormous brain cell. Kirk catches on immediately: these creatures are not separate animals, they are all parts of a single entity, connected in some mysterious fashion. This is how it resists phaser fire: each part draws strength from the whole.

McCoy's efforts to find some method to kill the creatures fail. Not heat, not radiation – nothing kills it. Kirk knows that if they cannot find a way to kill these creatures, he will be forced to destroy Deneva to prevent their spread. A million people will die if nothing can be done. Kirk cannot let the creatures spread, and has no wish to kill the Denevans, including his nephew. He demands a third alternative.

The key lies in exploring the properties of the sun. The Denevan was free of the creature moments before he died; something in the sun killed it. It is not radiation, it is not heat – could it be light? Kirk thinks it is. McCoy rigs a test cubicle, puts the sample creature inside, and confirms the theory: high intensity light is fatal to these creatures. Spock enters next; it is necessary to see what will happen to tissue that has infiltrated a victim. Spock volunteers to enter the cubicle - McCoy, who would prefer his guinea pig be someone other than the man he regards as Starfleet's best first officer, attempts to dissuade him, but Spock insists. This test does, in fact, succeed: the blinding light frees Spock of the creature and the pain – at the cost of his eyesight. Spock, exiting the cubicle, remarks that it is an equitable trade – the closest he comes to revealing how much pain he has been experiencing. Only then is the true tragedy revealed: lab tests indicate that the creatures are vulnerable only to a specific subset of the light spectrum: ultraviolet light is its Achilles' heel. McCoy is chagrined to realize that Spock need not have been blinded at all.

Despite this, the answer is at hand. Kirk orders satellite control to deploy a formation of 210 ultraviolet satellites at 72 miles altitude, in a permanent orbit. The satellites are turned on; the creatures begin to fail, to fall, to smoke and to die. Ground stations on Deneva quickly make contact; the creatures are dying everywhere.

Spock returns to the bridge – he can once again see. It seems that an inner eyelid, an hereditary trait of Vulcans, protected his eyes automatically. Relieved beyond words, McCoy asks Kirk not to mention his previous "best first officer" statement - at which point Spock turns around and thanks the doctor, and Kirk jokes that all the concern over Spock's eyes had led McCoy to forget about his ears.

"Captain's log, stardate 3287.2. The mass insanity we have tracked across this section of the galaxy seems to have already touched Deneva. That planet, colonized over a century ago, is one of the most beautiful in the galaxy."

"Captain's log, supplemental. Whatever the creatures are, they have apparently taken over all the inhabitants of Deneva. Meanwhile, ship's surgeon Dr. McCoy is examining a strange puncture wound left by one of the creatures on Mr. Spock's back."

"Captain's log, stardate 3289.8. I am faced with one of the most difficult decision of my life, unless we find a way to destroy the creatures without killing their Human hosts, my command responsibilities will force me to kill over a million people."

This is the only episode title of the Original Series that includes an exclamation point and, along with VOY: "Bride of Chaotica!", is one of only two episodes in the entire Star Trek franchise to incorporate that particular punctuation mark.

In addition, it is one of only three original series episodes in which the word "damn" is used, with McCoy cursing the "damnable logic" behind the experiments designed to destroy the creatures. The only other uses of the word "damn" occur in "Journey to Babel", wherein Kirk admits he can't "damn" Spock for his loyalty by taking command of the Enterprise at the cost of Sarek's life, and in "Court Martial" where Kirk makes a log entry stating that the evidence against him is "damning."

Deleted scene with Peter Kirk in captain's chair

A filmed scene cut from this episode featured dialog between Kirk and his nephew Peter. The dialog concerned Peter's returning to Deneva to live with Sam Kirk's partner. [1] The scene was edited from the episode.

Steven W. Carabatsos had an obligation in his contract, that he must deliver at least one script of his own while serving as story editor. Carabatsos left the series in late-1966, but still had to fulfill this task before departing. Having no idea of his own, Gene Roddenberry suggested him one, entitled Operation: Destroy!, which was the basis for this episode. (These Are the Voyages: TOS Season One)

The script, as originally conceived, did not envision the parasites being killed with light. Instead, the Enterprise leaves Deneva and destroys their home planet. By effectively annihilating the central "brain" that controls their operations, the ship renders the creatures harmless. Working from an early draft of the script, James Blish writes up this version in his adaption of the episode in his volume Star Trek 2.

The first draft script, entitled "Operation: Destroy", did not feature Kirk's brother and his family. In that version, a Denevan woman named Aurelan was in love with a young man named Kartan, who flew his ship into the Sun to destroy the creatures. Aurelan and her father were not infected, and helped the Enterprise crew in their research. (The Star Trek Compendium, p. 65)

The Deneva outdoor scenes were shot at the headquarters of TRW Space and Defense Park in Redondo Beach, California (currently the Northrop Grumman Space Technology headquarters). See [2] for the location in Google Earth. The establishing shot of Kirk's brother's lab was a building on the campus of UCLA, and the entrance of the building was the cafeteria at TRW. (Star Trek Encyclopedia 2nd ed., p. 112) The scenes at TRW were filmed 15 February1967.[3]

This is the first time McCoy's lab is seen. Inside the lab, the prop used previously as Balok's lamp device in The Corbomite Maneuver can be seen sitting on a shelf. Different components of sickbay were added over the first season, such as the decompression chamber seen in "Space Seed". McCoy's lab contains one of the life support canisters used on the Botany Bay.

The starmap showing the progression of the space madness also shows up on station viewscreens in future episodes.

The clubs used by the Denevans during their attack on the landing party appear to be thick lucite rods. Curiously, the gray, grooved clubs used by Spock during his fight with Kirk in "This Side of Paradise" and some of the miners on Janus VI in "The Devil in the Dark" were not recycled for this use.

Look closely when Kirk beams down alone to join the Landing Party. There is a presence moving about on the building horizon. Most likely a spectator caught in the presence of filming.

Unique to this episode, Uhura wears a black belt on the bridge, having worn one while on the landing party in the previous episode produced, "The City on the Edge of Forever". In "The Gamesters of Triskelion", she dons it when she joins the landing party and wears a communicator and phaser.

Although the ubiquitous jumpsuit-type outfits worn by the four Denevans who attack the landing party crop up in a variety of colors, this is one of the few in which a green one appears. There is also a red variety, apparently the only one ever seen in the original series.

There is a blooper from this episode in the first season blooper reel. The parasite creature that was supposed to hit Leonard Nimoy's back is seen hitting his backside instead, making Mr. Nimoy break up with laughter. Also appearing is a sequence showing the landing party with their phasers being used in tandem as shavers.

McCoy speaks about "getting the plates back" on Kirk's nephew to help with his medical treatment. This line references X-ray plates which were common in the 1960s but, in the 2260s, would have been extremely archaic and impractical compared to other medical advances of the time (such as DNA examination).

The Vulcan inner eyelid is mentioned again in ENT: "The Forge". In the Star Trek novel Spock's World, the inner eyelid was developed in one tribe of Vulcans who eventually took control over most of the planet.